Cell-Penetrating Anti-DNA Antibody for Radiosensitization and Cancer Therapy. Identification of a novel therapeutic agent that is selectively toxic to cancer cells or that sensitizes tumors to radiation or chemotherapy would be an important advance in cancer research. We have discovered that a cell- penetrating, anti-DNA antibody (3E10) can increase the sensitivity of cancer cells to ionizing radiation and to DNA-targeted chemotherapy agents both in cell culture and in vivo in mouse tumor models. Preliminary work suggests that 3E10 exerts its effects by blocking DNA repair. 3E10 was derived from a mouse model of systemic lupus erythematosus for treating that disease in humans. Importantly, the 3E10 monoclonal antibody, by itself, is synthetically lethal to BRCA2-deficient cancer cells, but is otherwise non-toxic to DNA repair- proficient cells in culture or to mice in vivo. The antibody also showed no detectable toxicity in humans when tested in a phase I trial in lupus patients. In preliminary work, 3E10 was found to greatly potentiate the effects of low-dose doxorubicin in BRCA2-deficient cells but not in BRCA2-proficient cells, providing a large therapeutic gain. We believe that the 3E10 antibody may be an effective therapeutic agent in the clinic, and we hypothesize that it will be particularly effective against cancers deficient in DNA repair, including certain malignancies of the breast, ovary, pancreas, and brain. The ability of 3E10 to penetrate into cells and nuclei distinguishes it from all other antibodies currently approved for cancer therapy; as such, 3E10 potentially represents a new class of cancer therapy agents. We will investigate the mechanism of action of 3E10 by probing its effects on specific DNA repair pathways via both cell-based and in vitro assays. We will define potential synthetic lethal interactions of 3E10 in DNA repair-deficient cancer cells, and we will study the effects of 3E10 in combination with radiation and other cancer therapies. These efforts will allow us to identify cancers that may be especially vulnerable to 3E10 and will provide the basis for designing optimized combination therapies in order to achieve therapeutic gain. We will also test the effectiveness of 3E10, alone and in combination, against tumors in vivo in mice using human tumor xenografts and orthotopic tumor models. In vivo assays to evaluate normal tissue effects will also be carried out. This work will provide key data to support the utility of 3E10 in cancer therapy, to identify opportunities for therapeutic gain, and to define initial effective means of use. Because 3E10 has previously been approved by the FDA for a phase I trial for treatment of lupus, there is already an established pathway to develop this agent for clinical use, but this time for cancer therapy. Consequently, we believe that the proposed work has a very high potential for translation into the clinic and so could have a direct and substantia impact on human health.